What is a Constant?
A constant is a value that cannot be changed after it is assigned.
Constants are useful when you have a value that should stay the same while the program runs, like the value of PI , the number of days in a week, or a fixed URL.
Unlike variables, which can be updated, constants keep the same value once they are created:
Runnable example
PI = 3.14159
print(PI)Why Use Constants?
Using constants makes your code easier to understand and maintain:
- Readability: Names like PI or DAYS_IN_WEEK explain what the number means.
- Safety: Constants protect important values from being accidentally changed.
- Easy updates: If you need to change a constant (like a tax rate), you only update it in one place.
Changing a Constant
Once a constant is assigned, trying to change it will usually cause an error or be ignored, depending on the language.
Runnable example
PI = 3.14159
PI = 3 # This works in Python, but it is not recommended
print(PI)Note
In Python, there is no built-in way to make a variable constant. The common practice is to write constant names in UPPERCASE to show other programmers that the value should not change.
Constants in Practice
Here are some common examples of constants:
- PI = 3.14159 (The mathematical constant pi)
- DAYS_IN_WEEK = 7
- URL = "https://example.com"
DAYS_IN_WEEK = 7
print("A week has", DAYS_IN_WEEK, "days")const DAYS_IN_WEEK = 7;
console.log("A week has " + DAYS_IN_WEEK + " days");final int DAYS_IN_WEEK = 7;
System.out.println("A week has " + DAYS_IN_WEEK + " days");const int DAYS_IN_WEEK = 7;
cout << "A week has " << DAYS_IN_WEEK << " days";